Method and apparatus for testing optical films

ABSTRACT

A structure for testing a luminescent film includes a Lambertian light source, an integrating sphere having an input port, and a measuring device. The Lambertian light source includes a mixing chamber having an input port and an output port, and a light emitter coupled to the input port. During testing the luminescent film is positioned between the output port of the mixing chamber and the input port of the integrating sphere. The measuring device is optically coupled to the integrating sphere.

This application claims the benefit or priority of and describesrelationships between the following applications: wherein thisapplication is a continuation in part of U.S. patent application Ser.No. 13/885,774, filed May 16, 2013, which is the National Stage ofInternational Application No. PCT/IB2011/055788, filed Dec. 19, 2011,which claims the priority of U.S. provisional application 61/425,805filed Dec. 22, 2010, all of which are incorporated herein in whole byreference.

BACKGROUND

1. Field of Invention

The present invention relates to a device and method for testing lightemitted by and transmitted through a luminescent film.

2. Description of Related Art

Semiconductor light-emitting devices including light emitting diodes(LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavitylaser diodes (VCSELs), and edge emitting lasers are among the mostefficient light sources currently available. Materials systems currentlyof interest in the manufacture of high-brightness light emitting devicescapable of operation across the visible spectrum include Group III-Vsemiconductors, particularly binary, ternary, and quaternary alloys ofgallium, aluminum, indium, and nitrogen, also referred to as III-nitridematerials. Typically, III-nitride light emitting devices are fabricatedby epitaxially growing a stack of semiconductor layers of differentcompositions and dopant concentrations on a sapphire, silicon carbide,III-nitride, or other suitable substrate by metal-organic chemical vapordeposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxialtechniques. The stack often includes one or more n-type layers dopedwith, for example, Si, formed over the substrate, one or more lightemitting layers in an active region formed over the n-type layer orlayers, and one or more p-type layers doped with, for example, Mg,formed over the active region. Electrical contacts are formed on the n-and p-type regions.

A light emitting device is often combined with one or more wavelengthconverting materials such as phosphors to create white light. All oronly a portion of the light emitted by the LED may be converted by thewavelength converting materials. Unconverted light emitted by the LEDmay be part of the final spectrum of light, though it need not be.Examples of common combinations include a blue-emitting LED combinedwith a yellow-emitting phosphor, a blue-emitting LED combined withgreen- and red-emitting phosphors, a UV-emitting LED combined with blue-and yellow-emitting phosphors, and a UV-emitting LED combined withblue-, green-, and red-emitting phosphors. Other wavelength convertingmaterials may be added to tailor the spectrum.

SUMMARY

It is an object of the invention to provide a device and method fortesting a luminescent film.

In embodiments of the invention, a structure for testing a luminescentfilm includes a Lambertian light source, an integrating sphere having aninput port, and a measuring device. The Lambertian light source includesa mixing chamber having an input port and an output port, and a lightemitter coupled to the input port. During testing the luminescent filmis positioned between the output port of the mixing chamber and theinput port of the integrating sphere. The measuring device is opticallycoupled to the integrating sphere.

A method according to embodiments of the invention includes positioninga Lambertian light source proximate a first surface of a luminescentfilm, positioning an opening in an integrating sphere proximate a secondsurface of the luminescent film, illuminating a portion of the film withthe Lambertian light source, and measuring a property of light from theluminescent film collected by the integrating sphere. In someembodiments, after measuring a property of light from the luminescentfilm, a property of a portion of the luminescent film is altered inresponse.

In embodiments of the invention, a structure for testing a luminescentfilm includes a light source, a light collection device, and a measuringdevice. During testing the luminescent film is positioned between thelight source and the light collection device. The measuring device isoptically coupled to the light collection device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one example of a device for testing the properties ofa luminescent film.

FIG. 2 illustrates the device of FIG. 1 including guiding rollers andionizing bars.

FIG. 3 illustrates an example of a device for testing the properties ofa luminescent film including a mixing chamber and collection optics.

FIG. 4 illustrates an example of a device for testing the properties ofa luminescent film including imaging optics and collection optics.

FIG. 5 illustrates a production line for making a luminescent film.

DETAILED DESCRIPTION

In accordance with embodiments of the invention, devices and method fortesting the properties of light from luminescent films are provided. Oneexample of a luminescent film is formed as follows: one or moreconventional powder phosphors are mixed with a binder such as acrylic orsilicone to achieve a target phosphor density. The phosphor/binder sheetis formed to have a target thickness, for example by spinning themixture on a flat surface or molding the phosphor sheet. Phosphor may bemixed with a binder in liquid form which is then cured or dried to forma flexible luminescent film. Another example of a luminescent film is apowder phosphor or other wavelength converting material that is sinteredinto a ceramic. Such a film may be sintered with the desired thicknessor may be sawn from a thicker ceramic phosphor. The luminescent film maybe flexible, as in the case of a phosphor/binder film, stretchable, orrigid, as in the case of a ceramic phosphor. Other wavelength convertingmaterials besides phosphors may be used, such as for example dyes,quantum dots, or optically-pumped semiconductor materials such as III-Vor II-VI materials. In the alternative the luminescent film may includelight scattering elements e.g. TiO_(x) or TiO₂ particles. In yet anotheralternative the film may not be an optical film and may include onlylight scattering elements e.g. TiOx or TiO₂ particles, without anywavelength converting materials i.e. without any phosphors, dyes,quantum dots, or optically-pumped semiconductor materials such as III-Vor II-VI materials.

After testing, the luminescent film may be attached or laminateddirectly to a suitable light source, or it may be spaced apart from thelight source, for example as part of a display. Examples of suitablelight sources include but are not limited to blue- or UV-emittingIII-nitride LEDs and laser diodes. Any other suitable light source maybe used with the luminescent films tested by the devices and methodsdescribed herein.

FIG. 1 illustrates a first device for testing the properties of lightfrom a luminescent film. In the device illustrated in FIG. 1, a smallarea of luminescent film 2 is illuminated by a Lambertian light source14 placed in close proximity to film 2 on one side of film 2. Forexample, the distance between port 13 of light source 14 and luminescentfilm 2 is less than 500 μm in some embodiments and less than 100 μm insome embodiments. Lambertian light source 14 includes a light emitter 3positioned in an opening in a mixing chamber 12. Light exiting themixing chamber at port 13 illuminates luminescent film 2. Light emitter3 may be, for example, a blue LED similar to an LED with whichluminescent film 2 is to be paired after testing, or any other suitablelight source. The peak wavelength of light emitter 3 may be matched tothat of a light source with which luminescent film 2 is to be pairedafter testing, in some embodiments. In any of the devices describedherein, the light emitter 3 or the entire light source 14 may betemperature controlled. Mixing chamber 12 may be, for example, a hollowsphere, the inside of which is coated with a highly reflectivescattering material. Port 13 may optionally be covered with atransparent window such as glass, sapphire, quartz, or plastic.

A device for measuring the light is positioned on the other side ofluminescent film 2. The measured light includes light emitted by thelight source 14 and scattered by luminescent film 2 at the samewavelength, and light absorbed by luminescent film 2 and reemitted overa different wavelength range.

Luminescent film 2 is positioned between a port 13 of mixing chamber 12and a port 17 of an integrating sphere 16. An integrating sphere is ahollow cavity with the interior coated with a highly diffuse reflectingmaterial to cause uniform scattering. Integrating spheres are known inthe art. One or both of port 13 and 17 may be knife-edge ports in someembodiments. In some embodiments, port 17 is larger than port 13, thoughthey may be the same size or port 17 may be smaller than port 13. Insome embodiments, the separation between ports 17 and 13 is no more than1 mm. In some embodiments, the separation between portions 17 and 13 issuch that luminescent film 2 is in sliding engagement with one or bothports. The surfaces of ports 17 and/or 13 in sliding engagement withluminescent film 2 may be electrically conductive.

Light captured by integrating sphere 16 may be coupled to a suitablemeasuring device 11 by, for example, a suitable light transmittingstructure 10 such as a fiber bundle. Alternatively, measuring device 11may be directly coupled to integrating sphere 16. Measuring device 11may be, for example, a spectrometer or a photo colorimeter. Measuringdevice 11 may measure properties of the captured light such as, forexample, the color, peak wavelength, full width at half maximum of thespectrum, total radiant flux, and/or luminous flux. Measuring device 11may also measure the ratio of scattered, unconverted photons toconverted photons. In some embodiments, the light source or a referencesource may emit long-wavelength light that is not wavelength-convertedby luminescent film 2 (for example, light at a peak wavelength greaterthan 650 nm in some embodiments) and measuring device 11 measures lightthrough luminescent film 2, in order to characterize scattering byluminescent film 2.

The device illustrated in FIG. 2 includes a Lambertian light source 14,integrating sphere 16, light transmitting structure 10, and measuringdevice 11 as in the device illustrated in FIG. 1. Light source 14 mayinclude a reference white light 22 which is used to monitor thestability of the instrument. Before testing a luminescent film 2, thespectra of one or both of source 3 and reference light 22 are measuredand compared to known values. In the device illustrated in FIG. 2, oneor more guiding rollers 19 are used to move and position luminescentfilm 2 between ports 13 and 17. Anti-static ionizing bars 18 may bepositioned proximate the measuring device 11 and/or integrating sphere16 to reduce tribocharging and to reduce the amount of dust in theambient. Integrating sphere 16 and/or mixing chamber 12 may includeoptional baffles 21 and 20, respectively, to improve light mixing ineach chamber. The device of FIG. 2 may include an optional calibratedsource 29 optically coupled to integrating sphere 16. Calibrated source29 is used to calibrate measuring device 11.

In some embodiments, integrating sphere 16 is positioned on a mechanismthat allows for removal and replacement without affecting alignment. Forexample, integrating sphere 16 may be mounted on a hinge which allowsintegrating sphere 16 to be lifted at the beginning of a production run.One end of a roll of a luminescent film 2 is placed over port 13, thenintegrating sphere 16 is brought back into its original position withluminescent film 2 disposed between port 17 and port 13. In someembodiments, integrating sphere 16 is positioned on kinematic ormagnetic mounts for ease of removal and reproducible replacement. Insome embodiments, one or both of integrating sphere 16 and mixingchamber 12 are mounted on springs which push ports 13 and 17 together inorder to maintain sliding contact of both ports with luminescent film 2.An advantage to the use of springs is that ports 13 and 17 can bedisposed in sliding contact with luminescent film 2 regardless of thethickness of luminescent film 2.

The devices illustrated in FIGS. 1 and 2 may have advantages over thedevices illustrated in FIGS. 3 and 4. In the devices of FIGS. 1 and 2,the luminescent film 2 is illuminated by a Lambertian source, as itwould be if combined after testing with a conventional LED as a lightsource. Substantially all the light emitted from the tested region ofluminescent film 2 is collected by integrating sphere 16, which mayimprove the accuracy of the measurement. In addition, vertical andtorsional displacements of luminescent film 2 may be minimized bymechanical constraints provided by ports 13 and 17, and by optionalguiding rollers 19. For example, in some embodiments, one or both ofports 13 and 17 are large-area flanges, wider than 25 mm in diameter insome embodiments. The flanges flatten luminescent film 2, therebyremoving any artifacts in light measurement having to do with unwantedchanges in illumination geometry. Further, the devices of FIGS. 1 and 2may be less sensitive to ambient light than the devices illustrated inFIGS. 3 and 4.

In the device illustrated in FIG. 3, luminescent film 2 is illuminatedas in the devices of FIGS. 1 and 2, by a Lambertian light source 14including a light emitter 3 coupled to a light mixing chamber 12 havinga port 13, which may be covered by a transparent window. All or aportion of the light from the portion of luminescent film 2 illuminatedby light from port 13 is collected by collection optics 7 in fieldaperture 8. The size of the sampled spot is determined by the size offield aperture 8. Light passes through field aperture 8 into ameasurement head 9, where it is then directed by a light transmittingstructure 10 to a measuring device 11, as described above in referenceto FIGS. 1 and 2.

In the device illustrated in FIG. 4, light from light emitter 3 isreimaged by imaging optics 4 such as for example a doublet pair, onto aspot 5 on luminescent film 2. Spot 5 is between 1 and 3 mm² in someembodiments. Wavelength-converted and unconverted scattered lightemitted from spot 5 is collected by collection optics 7 and directed tofield aperture 8. Each of imaging optics 4 and collection optics 7 maybe, for example, singlets made of fused silica or another UV-transparentmaterial. The size of field aperture 8 is selected such that only lightemitted by illuminated spot 5 is collected in field aperture 8, anopening in a measurement head 9. Light collected by measurement head 9is directed by a light transmitting structure 10 to a measuring device11, as described above in reference to FIGS. 1 and 2.

The axis of illumination of luminescent film 2 by light emitter 3 isnormal or substantially normal to the plane of luminescent film 2 insome embodiments. The illuminated beam is nearly collimated with anumerical aperture (N.A.) below 0.2 in some embodiments. Similarly, theaxis of collection optics 7 is normal or substantially normal to theplane of luminescent film 2 in some embodiments. In some embodiments,the acceptance cone created by collection optics 7 is narrow such thatthe collection N.A. is less than 0.2 in some embodiments and between0.05 and 0.15 in some embodiments.

In some embodiments, imaging optics 4 and light emitter 3, andcollection optics 7, aperture 8, and measurement head 9 are attached toa frame 1. Frame 1 is attached to a translation stage so frame 1 can bemoved to sample different parts of a stationary luminescent film 2. Insome embodiments, frame 1 is stationary and luminescent film 2 is moved,for example by rollers as illustrated in FIG. 2. In some embodiments, atesting device such as one of the structures illustrated in FIGS. 1-4 isa part of a luminescent film production line. The device may move inwidthwise direction with respect to the luminescent film, while theluminescent film moves through the production line.

The device illustrated in FIG. 4 can be modified for use with a largerilluminated spot 5. In such a device, collection optics 7 include a pairof lenses in a telecentric configuration. The pair of lenses areseparated by a distance such that their back focal planes coincide, andan angular aperture positioned in the back focal plane defines theacceptance cone.

In embodiments where the luminescent film 2 is stationary duringmeasurement, the probed area of luminescent film 2 is equal to the spotsize. In the devices of FIGS. 1, 2, and 3, the spot size is the size ofport 13; in the device of FIG. 4, the spot size 5 is determined byimaging optics 4. In some embodiments, the probed area is increased bymeasuring when the measurement apparatus (integrating sphere 16 orcollection optics 7) is moving relative to luminescent film 2, forexample as described above in reference to FIG. 4. The effective probedarea is approximately A=svτ, where s is the spot diameter, v is thevelocity of the relative motion of the luminescent film 2, and τ is themeasurement or integration time. The effective probed area can be variedby selecting the velocity and integration time.

FIG. 5 illustrates a device for producing a luminescent film targeted tospecific values of light conversion properties such as, for example,color point. In the production line illustrated in FIG. 5, theluminescent film includes a substrate film 23 that is coated with aluminescent layer 24 by any of a variety of methods known in the artsuch as, for example, application by a blade coater 25 as illustrated inFIG. 5, or by a spray process. The coated film may pass through one ormore optional post-coating steps 27, which may include, for example,drying, partial curing, application of a protective film, and/orbar-code marking. A tester 28 according to embodiments of the invention,which may be one of the devices illustrated in FIGS. 1-4, is placed atthe end of a luminescent film production line illustrated in FIG. 5.Tester 28 measures one or more properties of light from the finishedluminescent film, such as the color point. The properties of theluminescent film may be monitored only, to ensure adequate processcontrol, or the properties of the already-tested luminescent film and/orthe luminescent film remaining in the production run may be adjustedbased on measurements from tester 28 (and/or optional tester 26described below), in order to achieve desired properties in theluminescent film. For example, the thickness of the luminescentmaterial, the amount of luminescent material in the luminescent film, orthe coating parameters may be adjusted based on measurements from tester28 and/or tester 26.

In some embodiments, an additional optional tester 26 according toembodiments of the invention, which may be one of the devicesillustrated in FIGS. 1-4, is placed downstream from coater 25. Anexperimentally pre-determined relationship between measurements oftesters 26 and 28 is used to convert a color target for the final filmas measured by tester 28 into the corresponding color target for tester26. The reading by tester 26 is brought to a target value by adjustingsuitable parameters of the coating process, at the beginning of orduring a luminescent film production run. Tester 26 may be a testeraccording to an embodiment of the present invention, or may be adifferent type of tester whose target reading will be calculated fromits pre-determined relationship to the spectral measurement of thefinished luminescent film by tester 28. Examples of properties measuredby tester 26 include but are not limited to absorbance at a particularwavelength in a transmission or reflection mode, light scatteringsignature, film thickness, light interferometric profile, and/orelectrical impedance.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. Therefore, it is not intended that the scopeof the invention be limited to the specific embodiments illustrated anddescribed.

What is being claimed is:
 1. A system for testing a film, the systemcomprising: a Lambertian light source, the Lambertian light sourcecomprising a mixing chamber having an input port and an output port; alight emitter coupled to the input port; an integrating sphere having aninput port; at least one baffle disposed in one of the mixing chamberand the integrating sphere; and a measuring device for producing ameasurement; wherein during testing the film is positioned between theoutput port of the mixing chamber and the input port of the integratingsphere, wherein the measuring device is optically coupled to theintegrating sphere, wherein a property of a portion of the film isaltered in response to the measurement.
 2. The system of claim 1 whereinthe film comprises light scattering particles.
 3. The system of claim 2wherein the light scattering particles comprise TiOx particles.
 4. Thesystem of claim 1 wherein the film comprises luminescent materials. 5.The system of claim 4 wherein the luminescent materials comprisephosphors.
 6. The system of claim 1 wherein the output port of themixing chamber is spaced less than 500μ from the film.
 7. The system ofclaim 1 wherein the output port of the mixing chamber is spaced lessthan 1 mm from the input port of the integrating sphere.
 8. The systemof claim 1 wherein during testing the film is in sliding engagement withat least one of the input port of the integrating sphere and the outputport of the mixing chamber.
 9. The system of claim 1 wherein themeasuring device is, one of a spectrometer and a photo colorimeter. 10.A structure for testing an optical film, the structure comprising: afirst tester, the first tester comprising: a Lambertian light source,the Lambertian light source comprising a mixing chamber having an inputport and an output port; a light emitter coupled to the input port; anintegrating sphere having an input port; at least one baffle disposed inone of the mixing chamber and the integrating sphere; and a measuringdevice; and a second tester disposed downstream relative to the opticalfilm of the first tester, wherein during testing the optical film ispositioned between the output port of the mixing chamber and the inputport of the integrating sphere; wherein the measuring device isoptically coupled to the integrating sphere wherein the second testermeasures a property of the optical film wherein the optical filmcomprises light scattering particles.
 11. The structure of claim 10wherein the light emitter is an LED configured to emit white light. 12.The structure of claim 10 wherein the measuring device is one of aspectrometer and a photo colorimeter.
 13. The structure of claim 10wherein during testing, the output port of the mixing chamber is spacedless than 500 μm from the optical film.
 14. The structure of claim 10wherein during testing, the output port of the mixing chamber is spacedless than 1 mm from the input port of the integrating sphere.
 15. Thestructure of claim 10 wherein during testing the optical film is insliding engagement with at least one of the input port of theintegrating sphere and the output port of the mixing chamber.
 16. Thestructure of claim 10 wherein at least one of the input port of theintegrating sphere and the output port of the mixing chamber is aknife-edge port.
 17. The structure of claim 10 wherein the luminescentfilm comprises phosphor disposed in an optical material.
 18. A methodcomprising: positioning a Lambertian light source proximate a firstsurface of a optical film; positioning an opening in an integratingsphere proximate a second surface of the optical film; illuminating aportion of the optical film with the Lambertian light source; measuringa property of light from the optical film collected by the integratingsphere; and after measuring a property of light from the optical film,altering a property of a portion of the optical film in response to themeasurement wherein the optical film comprises light scatteringparticles.
 19. The method of claim 18 wherein the light from the opticalfilm comprises light from the Lambertian light source transmittedthrough the optical film at the same wavelength and light from theLambertian light source absorbed by the optical film and emitted by theoptical film at a different wavelength.
 20. The method of claim 18wherein the scattering particles comprise TiO_(x).